That strong infrared emission is common in spiral galaxies became
clear at the time of publication of the first group of IRAS papers.
De Jong et al. (1984)
examined a sample of 165 galaxies in the
Shapley-Ames catalog and found that over 80% of spirals (Sa and later)
were detected by IRAS, though none of the ellipticals were seen. Since
the publication of the IRAS point source catalog it has been possible
to examine the incidence of infrared emission from galaxies on a
proper statistical basis. The infrared luminosity function has been
calculated by several groups
(Lawrence et al. 1986;
Soifer et al. 1986;
Rieke and Lebofsky 1986).
In general it resembles that of
galaxies at visible wavelengths except that there appears to be an
excess of sources with luminosities above about 1010
L
. These high
luminosity objects include active galaxies and ``starburst'' galaxies
that will be receiving much attention elsewhere in this volume. I will
generally be discussing lower-luminosity spiral galaxies in this
paper, although I will present evidence that a number of early-type
barred spiral galaxies may be exhibiting signs of low-level nuclear activity.
A crucial problem in trying to use infrared luminosities as a guide to star formation activity in galaxies is the difficulty of distinguishing emission from the diffuse interstellar medium and emission from dust heated by newly formed or still-forming stars. Attempts have been made to use various flux ratios for this purpose. de Jong at al. (1984) found that the ratio LIR / LB increases with the 60-100 µm color temperature. From this result has come the idea that spiral galaxy disks contain a cool dust component that corresponds to the diffuse or ``cirrus'' emission, plus a warmer component that dominates in cases of galaxies that are undergoing large amounts of star formation.
The separation of the infrared emission into a ``cirrus'' and a star
formation component may be tested by comparing the IRAS data with some
independent parameter, such as the
H
or the CO fluxes. The results
are not conclusive. Two groups have compared the IRAS fluxes from a
number of spiral galaxies with the star formation rates calculated by
Kennicutt from measurements of
H spectrophotometry;
Moorwood, Veron-Cetty,
and Glass (1986)
conclude that the infrared emission from
Kennicutt's galaxies matches that expected on the basis of
H emission
from star formation regions, but
Persson and Helou (1986)
conclude
that the bulk of the infrared emission from these galaxies comes from
the interstellar ``cirrus'' component.
Young (1986)
finds a correlation
between infrared luminosity and the luminosity in the 2.6 mm CO line,
supporting the idea that much of the infrared emission is associated
with star formation. Since some of the galaxies in her study are of
very high luminosity, however, the result does not necessarily apply
to ``normal'' galaxies.
A challenge to the idea that the 60-100 µm color temperature is an indicator of the role of star formation in galaxies has been raised by Burstein and Lebofsky (1986). They present evidence that the apparent far-infrared luminosity of Sc galaxies varies with inclination. This result would imply that the disks of these galaxies are optically thick at 100 µm, which in turn would require that the emission be concentrated within the central 1 kpc diameter region. Burstein and Lebofsky's conclusions are disputed by Rice, Elias, and Persson (1986), who point out that problems arise due to the difficulties of correctly classifying galaxies that have high inclinations. Another difficulty with Burstein and Lebofsky's hypothesis is that Devereux, Becklin and Scoville (1987) found that the emission from most Sc galaxies is not concentrated within the central 1 kpc, at least at 10 µm.
The strongest correlation that has appeared so far from the IRAS data is the very close proportionality between the 60 µm flux density from warm dust and the centimeter-wavelength nonthermal emission (de Jong et al. 1985; Helou, Soifer, and Rowan-Robinson 1985). This correlation has not yet been satisfactorily explained. One popular model has it that the radio emission is dominated by synchrotron emission from individual supernova remnants and that both the infrared luminosity and the supernova rate are proportional to the star formation rate. A problem with this model is that in the two cases where adequate data are available, namely M82 (Kronberg et al. 1985) and the Galaxy, individual supernova remnants are responsible for only a small fraction of the total synchrotron emission from the galaxy disk. In an alternative model the radio emission originates from relativistic electrons in the general interstellar medium. The difficulty with this model is that synchrotron emission depends strongly on the magnetic field strength as well as the number density of relativistic particles. To explain the proportionality of infrared and radio emission it is necessary to identify a mechanism that controls the interstellar magnetic field strength in various different star-forming regions. As yet no such mechanism has been identified.